There are several types of subsea fibre-optic penetrators. One possible method of forming a fibre-optic penetrator is by encapsulating the fibre in an external tube, typically a metal tube, by means of a potting compound, typically an epoxy compound, which bonds so well to both fibre and tube that it makes a watertight connection. The fibre-optic penetrator is then provided with external sealing elements on the outside of the external tube and inserted into a cylindrical hole in the wall which has to be penetrated by the penetrator. The sealing elements in turn provide a seal between the walls of the hole and the surface of the tube, thereby achieving a total seal of the wall feed-through. Alternatively, the penetrator may consist in the fibre being directly encapsulated in a cylindrical potting compound without the use of an external tube, or in a further alternative the fibre, which in this case must be surrounded by a plastic sheath of a certain thickness, forms a seal directly against the penetrator wall with rubber sealing, typically an O-ring. Glass-cast fibre-optic penetrators are found for land-based equipment.
During use subsea penetrators are subjected to substantial stresses and have completely different operating conditions from, for example, a corresponding penetrator onshore. A subsea penetrator must provide a good seal even with great variations in both pressure and temperature.